In recent years, there has been a global trend of improving the composition of end consumer products such as cosmetic products to products which do not contain polyoxyethylene as a whole. For example, in Germany, there is an increasing demand to replace raw materials containing polyether groups with non-polyether raw materials that do not contain polyether groups in response to negative perceptions stemming from examinations by consumer-oriented information-providing magazine companies regarding the safety of products containing polyoxyethylene (PEG). In addition, in South Korea, there is an increasing interest in non-polyether-based silicone surfactants due to concerns that since formalin can be produced by the oxidative degradation of polyoxyethylene (PEG), products containing polyoxyethylene may cause skin irritation.
In concert with the global trends described above, there is a demand to evolve technology in the field of silicone-based surfactants from conventional polyether-modified silicone to non-polyether-based silicone. Further, glycerin-modified silicone has attracted attention as a surfactant having enhanced safety since the oxidative stability of glycerin-modified silicone is considered to be superior to that of polyether-modified silicone.
However, conventional glycerin-modified silicone has a substantial problem. This problem is that even if there was a desire to use a conventional glycerin-modified silicone as an emulsifier for a water-in-oil emulsion, it could not withstand use in the actual formulation of a cosmetic composition since its emulsification capacity, its ability to accommodate various oil agents, and the like are low. Therefore, it has been unavoidable to use polyether-modified silicone, which is a more reliable emulsifier, in combination with glycerin-modified silicone, and it has not been possible to achieve the goal of improving cosmetic compositions to formulations not containing polyoxyethylene (also called “PEG-FREE” hereafter). Moreover, there has not been any reports regarding materials suitable as thickening emulsifiers for a water-in-oil emulsion in technologies related to conventional glycerin-modified silicone (Patent Documents 1 to 6).
On the other hand, research focusing on diglycerin-modified silicone is scarce, such as that appearing in Patent Document 7. However, the glyceryl ether-modified organo(poly)siloxane having a specific structure reported therein has poor compatibility with various organic oils, and a stable water-in-oil emulsion cannot be obtained with a formulation having a high ratio of organic oil in the oil phase. This leads to the problem that separation is induced over time or due to temperature. Therefore, it has not been possible to use the substance alone as an emulsifier in such formulations, and it has been necessary to use the substance in combination with long-chain alkyl/polyether-co-modified silicone or the like.
In Patent Document 8, the present applicant therefore proposes using a co-modified organopolysiloxane copolymer having a group having a carbosiloxy dendron structure and a hydrophilic group such as glycerin or polyhydric alcohol in a molecule as a surfactant, powder treatment agent, or a surface treatment agent that can be suitably used in the cosmetic field, in particular. In particular, in Practical Example 13 of Patent Document 8, the present applicant proposes a novel glycerin derivative-modified silicone No. 13 having a group having a siloxane dendron structure, a tetraglycerin derivative group, and a diglycerin derivative group in each molecule, and proposes a water-in-oil emulsion composition (Formulation Example 5) and an external skin preparation in the form of a water-in-oil emulsion (Formulation Example 33) thereof. Although the glycerin derivative-modified silicone proposed here makes it possible to prepare a particularly stable water-in-oil emulsion when an oil phase is the silicone oil main component, an emulsion of low viscosity tends to be obtained. In addition, emulsion itself is difficult for a system in which a non-polar organic oil with a relatively large molecular weight such as a mineral oil serves as the main component of the oil phase. There still remains room for improvement regarding the emulsion stability thereof—in particular, the long-term storage stability at high temperature.
Moreover, in Patent Document 9, the present applicant has reported that among the co-modified organopolysiloxanes described in Patent Document 8, specific co-modified organopolysiloxanes in which the average degree of polymerization of the glycerin portion is in a range of 3 to 5 are particularly useful as thickening agents or gelling agents for oil-based raw materials. However, when using these materials in an emulsion system, the stability of the emulsion can be ensured only by thickening or structuring the oil phase, and the emulsion particle size itself is as large as in the case of conventional glycerin-modified silicones. Therefore, there still remains room for improvement from the perspective of the emulsion capacity or the perspective of the reliability as an emulsifier.